union.md
In S7: An Object Oriented System Meant to Become a Successor to S3 and S4

has useful print method

Code
  foo1 <- new_class("foo1", package = NULL)
  foo2 <- new_class("foo2", package = NULL)
  new_union(foo1, foo2)
Output
  <S7_union>: <foo1> or <foo2>

base unions display as expected

Code
  class_vector
Output
  <S7_union>: <logical>, <integer>, <double>, <complex>, <character>, <raw>, <expression>, or <list>
Code
  str(class_vector)
Output
  <S7_union>: <logical>, <integer>, <double>, <complex>, <character>, <raw>, <expression>, or <list>